Conventionally, as an example of a commonly used backlight device for liquid crystal displays, there is known a direct-type backlight device which is provided, in this order, with a reflecting plate, a plurality of linear light sources (e.g., cold cathode tubes) arranged in approximately parallel, and a light diffusing plate acting as a illuminating surface by diffusing and then emitting direct light from these linear light sources and reflected light on the reflecting plate. On the illuminating surface of such a direct-type backlight device, there is a tendency that the position right above the linear light source has high luminance, and the luminance becomes lower as leaving from the right-above position, which sometimes causes the periodic luminance unevenness.
Thus, for example, Patent Document 1 (JP Hei-6-273760-A) discloses a technique for relatively increase the light quantity irradiating the part between the linear light sources by printing a light quantity correcting pattern in a form of stripes or dots on the light diffusing plate to thereby reduce the light quantity irradiated to the region right above the linear light source. However, in this method, an availability of the light quantity irradiated from the linear light source is reduced and the sufficient luminance is not obtained because a part of the light quantity is blocked by the light quantity correcting pattern.
In recent years, a point light source such as LEDs (light emitting diodes) has been also utilized in place of the linear light source such as cold cathode tubes because such a point light source can give a sufficient luminance with reduced electric power consumption. However, even if the point light source is used as the light source of the direct-type backlight device, there still was a problem of the luminance unevenness occurring on the illuminating surface as was in the prior art. Addressing thereto, for example, Patent Document 2 (Utility Model Registration No. 3114467) discloses provision of a reflecting plate, a plurality of point light sources and a light diffusing plate, wherein prism-shaped light guide pyramids are formed on both a light receiving surface and a light emitting surface of the light diffusing plate, aiming at reducing the luminance unevenness on the illuminating surface by this light guide pyramids.